I've designed them, installed them and done a lot of failure analysis on them. I have never had a system I designed fail, and I don't use a make-up water system at all, including the one at my house. I am currently working on three (lawyers involved and all) where the tubing froze, likely because the make-up water system admitted water, but there was no glycol added. Now a premixed glycol feeder, on a large system is not a big deal. In any event, a closed system with an expansion tank is a better solution. Glycol in the system does not significantly reduce the efficiency and actually keeps the system in better shape overall.

Depending on the tubing type and encasement, some can freeze and not have any issues, but you still don 't want to risk it. If it was me, I would delete the water make-up, ensure the boiler and heating loop have pressure expansion and relief and call it good. If you have an alarm system, add a low temp alarm sensor for some added peace of mind.